DESCRIPTION (Applicant's abstract): The overall objective of this application is to define the molecular organization of myelin, the diverse adhesive mechanisms that stabilize its multilamellar sheath, and what defects in its organization and molecular constituents may lead to dysmyelination or demyelination such as occur in certain peripheral neuropathies and in multiple sclerosis. The approach is based on a correlation of results from X-ray crystallography and solution scattering, membrane diffraction, and electron microscopy. The specific aims, which are focused on the structure and role of the major transmembrane protein of peripheral nerve myelin (P0-glycoprotein) are: (1) to determine the three-dimensional structure of P0-glycoprotein for human and Xenopus. The proteins analyzed will include recombinant molecules having the native amino acid sequence, as well as those having specific sequence alterations known to occur in human peripheral neuropathies. These studies will inform about the atomic structure of P0 and about the crystal contacts or adhesion interfaces that may be responsible for the role of this protein in myelin formation and stability. (2) To characterize the protein-protein interactions between nearest-neighbor P0 molecules in a membrane mimetic environment using small-angle X-ray scattering. The membrane protein will be solubilized in aqueous solutions of detergents at very low concentration, and solution scattering will be undertaken using a synchrotron X-ray source. These studies will provide information about the interprotein molecular contacts that P0 molecules make in a milieu that more closely resembles its native environment (the lipid bilayer of the myelin membrane) than does a crystal. (3) To evaluate the membrane-membrane interactions in myelin of Xenopus peripheral nerves. Determining the pH- and ionic strength-dependence of membrane structure and packing in dissected peripheral nerves that have been incubated at different will provide strong constraints for testing hypotheses about the adhesion mechanisms of P0 at both the cytoplasmic and extracellular membrane appositions. This hierarchy of experimental objectives will allow the investigator to uniquely correlate structural data from the atomic to the molecular, to the membrane level, and thus contribute to an understanding of the structural biology of a membrane protein that figures significantly in both health and disease.